As a means of forming a thin film of a metal platinum, heretofore, there has been known a method of coating a platinum paste of a so-called liquid platinum, which consists essentially of a reaction product of a balsam sulfide and a platinum halide, on a substrate followed by heating it to form a thin platinum film on the substrate.
As examples of other platinum pastes, JP-A-64-177 mentions a platinum alkyl mercaptide; JP-A-64-178 mentions a platinum pinene mercaptide; and JP-A-64-179 mentions a platinum essential oil mercaptide-containing composition. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
JP-A-51-86451 mentions a method of coating a bis(cis,cis-cycloocta-1,5-diene) platinum solution on a carrier followed by heating the carrier so as to decompose bis(cis,cis-cycloocta-1,5-diene) platinum on the carrier to thereby deposit a metal platinum on the same.
It further mentions a method of coating an organic platinum complex of a general formula (II): EQU Pt(Q).sub.3 ( II)
where Q represents a compound having at least one olefinic double bond or acetylenic triple bond, such as an olefin, an allene, an acetylene or a substituted olefin, on a carrier followed by heating the carrier so as to decompose the compound of the above-mentioned formula (II) to thereby deposit a metal platinum on the same.
In the above-mentioned method of using a paste consisting essentially of a reaction product of a balsam sulfide and a platinum halide which contains a large number of sulfur atoms and halogen atoms, a large amount of corrosive gases of sulfur oxides, halogens or hydrogen halides are generated during heating of the carrier for forming the platinum coat. As a result, the thus generated corrosive gases would corrode the firing furnace. Additionally, where a thin film of a metal which is easily corroded by such gases, such as silver or the like, has already been formed on the substrate, the thin film would also be corroded.
Regarding the methods illustrated in the above-mentioned JP-A-64-177, 64-178 and 64-179, all the compounds which are used as a platinum source contain sulfur atom and therefore generate also a large amount of corrosive sulfur oxides during heating of the coated film.
Regarding the method as illustrated in the above-mentioned JP-A-51-86451, in which a bis(cis,cis-cycloocta-1,5-diene) platinum solution is coated on a carrier and thereafter the thus coated carrier is heated so as to deposit a platinum metal on the carrier, a paste having a high concentration could not be obtained as the solubility of bis(cis,cis-cycloocta-1,5-diene)platinum to the solvent is low. Therefore, there is a limitation on the elevation of the content of platinum in the solution to be coated and, as a result, the thickness of the platinum film to be coated is naturally limited. In addition, selection of additives which are added to the coating solution for the purpose of improving the coatability of the solution and of improving the adhesiveness between the platinum film formed and the carrier, as well as the amount of such additives to be added are also extremely limitative, in carrying out the method. A satisfactory thin film of platinum metal could not be obtained from a solution of bis(cis,cis-cycloocta-1,5-diene)platinum. Regarding the other method as illustrated in the same JP-A-51-86451, in which a compound of the above-mentioned general formula (II) is coated on a substrate and thereafter the thus coated substrate is heated so as to decompose the compound of the formula (II) to thereby deposit a platinum on the substrate, a paste having a constant quality could not be obtained since the solution of the compound of the above-mentioned formula (II) is unstable and the solution is easily decomposed during storage even at room temperature. The published specification of JP-A-51-86451 is silent on the solubility of bis(cis,cis-cycloocta-1,5-diene) platinum and that of platinum compounds of the above-mentioned general formula (II), and it is also silent on a paste containing the above-mentioned platinum compound for forming a thin film of a metal platinum by pyrolysis. Additionally, it does not refer to platinum complexes of a type containing two Q's as ligands.
In particular, in the technical field of forming electroconductive circuits on a substrate, a method where a solution of an organic platinum compound is coated on a substrate and thereafter the coated film is heated and fired to form such an electroconductive thin film that having a thickness of about 0.1 to 1 micron is desired.